An image forming apparatus such as a laser printer is mounted with a developing device having a developing chamber provided with a developing roller. JP-A-9-319202 describes a related art developing device demountably mounted with a developer cartridge containing a developer.
In the related art developing device, a developer supply opening is formed at a center of the developer cartridge and a pair of developer receiving openings is formed on both sides thereof. An agitation member for agitating the developer is disposed in the developer cartridge.
The developer in the developer cartridge is supplied from the developer supply opening to the developing chamber by the agitation member. In contrast, the developer staying in the developing chamber is returned from the developer receiving opening to the developer cartridge. Accordingly, in the related art developing device, the developer is circulated in the developing device.